Water density varies with temperature, and the curve is not a straight line near ordinary room conditions. This table gives air-free liquid-water density calculated at 2.5°C intervals from 0°C through 40°C using the NIST recommended correlation. The density is shown in kilograms per cubic meter at the stated reference pressure.

Use the correct temperature range and units

NIST recommends the correlation used here for water from 0°C to 40°C. Enter temperature in degrees Celsius, and the equation returns density in kg/m³. The maximum density is near 4°C, which is why density first rises and then falls as the water warms across this range. The values in the table are rounded to 0.001 kg/m³; the extra digits do not represent the accuracy of a particular thermometer, water sample, or weighing system.

These calculated values are for air-free water. NIST notes that laboratory water is commonly air-saturated and provides a separate correction for that condition. Dissolved air contributes a small density difference that can matter in high-accuracy gravimetric work. Tap water, saline water, process fluids, and samples with contaminants do not have the same density as pure air-free water.

Pressure and phase matter

The table identifies a reference pressure of 0.1 MPa and applies only to liquid water in the stated temperature interval. Pressure changes density, although the change is small for many ordinary applications. Near the boiling point, phase and pressure conditions need explicit attention; this table should not be extrapolated to steam, superheated water, freezing conditions, or other pressure ranges.

For a simple volume-to-mass conversion, multiply the measured volume in cubic meters by density in kg/m³. Keep volume units consistent: one liter is 0.001 m³. For example, a 1-liter sample near 20°C has a mass near 0.998 kg under the air-free reference assumption. A real measurement may differ because of dissolved gas, impurities, container expansion, calibration, temperature gradients, and the actual pressure.

Choose a method that matches the precision

For a rough estimate, interpolate between nearby table values and state the assumed condition. For calibration or high-accuracy gravimetry, use the full NIST method, account for the sample’s air content, and follow the applicable measurement procedure. Measure the water temperature where the density is relevant, allow the sample to equilibrate, and record instrument uncertainty. Do not treat a rounded lookup table as a substitute for a traceable calibration calculation.

If the sample is not pure water or is at an unusual pressure, use a validated property source for that composition and state. For engineering work, confirm whether the application needs density at the actual pressure or a standard reference condition. This dataset provides a reproducible reference curve and does not certify a particular fluid sample or weighing result.

Interpolation should be used with care when measurement uncertainty is small or a value lies near the density maximum. A straight-line interpolation is a convenient estimate between neighboring entries, but the source correlation can be evaluated directly for a temperature with more precision. Preserve the input temperature and calculation method in a calibration record. The table’s increment and rounding are intended for quick reference, not as a claim that every sample has the displayed density exactly.

For example, the correlation evaluates to approximately 998.2 kg/m³ at 20°C, so a liter of air-free water has a reference mass near 0.9982 kg. At about 4°C, the density is near 999.97 kg/m³. This behavior is one reason to use the temperature-specific value instead of assuming water is exactly 1000 kg/m³ for precision work. Keep the actual temperature attached to any converted volume or mass measurement.

Source: NIST GLP 10, Water Density Calculations. Dataset values are calculated from NIST’s air-free correlation, whose stated recommended range is 0°C to 40°C.